The definite presence or absence of hydrocarbons in formations beneath the earth surface can only be established by drilling and it provides the exact estimate of the reservoir, its pressure and size. But drilling is very costly and oil companies will wish to drill only at the sites with high probability of finding oil or natural gas. Hence, it is essential, economically and ecologically, that geological, geochemical and geophysical studies are carried out beforehand.
Geophysical studies consist of making measurements of fundamental physical data, the gravitational field, magnetic fields and electrical resistance as the function of depth and interpreting these results in geological terms.
Seismic surveying involves making two or three dimensional ultrasound image of the earth by analyzing the pattern of propagation of waves through earth. This image of the subsoil provides the information of subsurface structure.
Sound waves are transmitted into the subsoil where they propagate through the rock mass and in their way undergoing reflection and refraction at certain geological discontinuities. The reflected waves which return to the surface are recorded by sensors.
Seismic Surveying on Land
For surveying on land the seismic waves used are composed of tremors on the ground surface artificially generated by buried explosives, drilled into the ground or by Thumper trucks, which slam heavy plates on the ground.
The seismic waves travel beneath the surface of the Earth and are reflected back by the various rock layers. The reflections are received by geophones which are distributed at the surface in definite geometric configurations. Geophones convert the vibrations received from the ground into electrical signals. The electrical signals are supplied to the recording truck, connected to geophones, which logs the acquired data.
Offshore Seismic Surveying
Offshore surveying team consists of two crews on board the exploration vessel. One crew is for normal navigational operations and the other crew carries out the seismic measurements. Waves for surveying are generated by compressed air guns which shoot pulses of air into water. Hydrophones, contained in a tube called streamer, are towed behind the vessel to receive the reflected waves.
Seismic surveying at sea
Seismic data interpretation
The seismic waves travel at different speeds depending upon the type or density of rock layers through which they pass. Return time, the interval in which the signal is returned, when converted into depth provides the possible description of the subterranean structures. These results of a seismic survey can indicate the presence of possible hydrocarbon traps, which would be the target of drilling.
Although modern oil-exploration methods are better, they still may have only a 10-percent success rate for finding new oil fields. Preliminary assessments are confirmed only when a borehole has been drilled. Next section takes up the oil well drilling.